Fix error output in JSON mode

cmd/kbld/kbld.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"fmt"
 	"io"
 	"log"
 	"math/rand"
@@ -26,11 +27,17 @@ func main() {
 	confUI := ui.NewConfUI(ui.NewNoopLogger())
 	defer confUI.Flush()
 
-	command := cmd.NewDefaultKbldCmd(confUI)
+	options := cmd.NewKbldOptions(confUI)
+	command := cmd.NewKbldCmd(options)
 
 	err := command.Execute()
 	if err != nil {
-		confUI.ErrorLinef("kbld: Error: %s", uierrs.NewMultiLineError(err))
+		multiLineErr := uierrs.NewMultiLineError(err)
+		if options.UIFlags.JSON {
+			_, _ = fmt.Fprintf(os.Stderr, "kbld: Error: %s\n", multiLineErr)
+		} else {
+			confUI.ErrorLinef("kbld: Error: %s", multiLineErr)
+		}
 		os.Exit(1)
 	}
 

cmd/kbld/kbld_test.go

@@ -0,0 +1,52 @@
+// Copyright 2026 The Carvel Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const failureExitCode = 1
+
+func TestJSONErrorsAreWrittenToStderr(t *testing.T) {
+	cmd := exec.Command(os.Args[0], "-test.run=^TestKbldProcess$", "--",
+		"--json", "-f", filepath.Join(t.TempDir(), "missing.yml"))
+	cmd.Env = append(os.Environ(), "KBLD_TEST_PROCESS=1")
+
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	err := cmd.Run()
+	var exitErr *exec.ExitError
+	require.ErrorAs(t, err, &exitErr)
+	assert.Equal(t, failureExitCode, exitErr.ExitCode())
+	assert.Empty(t, stdout.String())
+	assert.True(t, strings.HasPrefix(
+		stderr.String(), "kbld: Error: Unable to stat file:"))
+}
+
+func TestKbldProcess(t *testing.T) {
+	if os.Getenv("KBLD_TEST_PROCESS") != "1" {
+		return
+	}
+
+	for i, arg := range os.Args {
+		if arg == "--" {
+			os.Args = append([]string{"kbld"}, os.Args[i+1:]...)
+			main()
+			return
+		}
+	}
+
+	t.Fatal("missing command separator")
+}